1. Field of the Invention
The present invention relates to a discharge monitoring system for monitoring levels of volatile organic constituents (VOCs) in for example, fluids such as air streams, ground water, soils or process effluent liquids and gases, and more particularly to a monitoring system to be used on site.
2. Description of the Related Art
Groundwater contaminants, such as gasoline from leaking storage tanks and solvents from commercial/industrial waste sites, by law require remediation by the property owner. As described in U.S. Pat. No. 4,892,664, the method of clean-up generally involves the use of a water stripping tower to bring polluted groundwater to the surface in a continuous process of cleaning and recirculation back into the ground. The operation of the stripping tower system is primarily open loop with no built-in checks of the effluent water or other tower parameters to indicate proper operations after initial system installation. Government agencies regularly inspect the efficiency of the cleaning process via a water sample analysis to assure that the stripping tower effluent meets local regulatory specifications for discharge back into the water table. Should an inefficiency be identified, the property owner is liable for substantial daily fines until the condition is corrected.
It is in the property owner's best interest to know on a continuous basis the condition of the tower's effluent. However, systems presently used for this purpose are so costly to purchase, operate and maintain that only major corporations and government agencies themselves can afford to use them.
Tin-dioxide sensors have been commercially available in gas alarm applications since the early 1970s. See U.S. Pat. No. 4,236,138. Their use has been limited to high concentration detection because of the many interferences the sensors are susceptible to. These interferences include:
1) cross-sensitivity to different VOCs; PA1 2) non-selectivity to almost any volatile organic compound; PA1 3) temperature and humidity changes; and PA1 4) changes in flow velocity across the sensor. PA1 In addition to the above, the conventional tin-dioxide sensor has not been applied to analytical applications specifically for the following reasons: PA1 1) non-linearity and low sensitivity with respect to conventional laboratory detection methods such as Flame or Photo Ionization, Electro-Chemical, Electron capture, Mass Spectrometry and other mass measuring techniques like Surface Acoustic Wave (SAW); PA1 2) sensor drift and/or lack of long-term stability; PA1 3) response varies from sensor to sensor, a characteristic of thick film manufacturing; and PA1 4) the requirement of an oxygen-rich environment to operate the sensor. Most gas chromatographs operate on helium or nitrogen.